A large number of auditory studies explored the role of function words in syntactic processing, but few researched function words in written input. The present study probed the role of function words in word skipping, sentence compacting, chunking preference and the detection mechanism of grammatical incongruence by means of number estimation across 4 syntactic conditions (grammatical sentences, scrambled sentences, sentences with agreement errors and sentences with structural errors), 3 sentence lengths (6 or 7 words, 8 or 9 words, 10 or 11 words) and 3 ratios of function words and content words. We find that appropriate usage of function words highly facilitates syntactic analysis though function words are always skipped in proficient reading. Adjacent function words and content words in grammatical sentences are more likely to be processed as chunks, and this effect of chunking make sentences significantly more compact than scrambled sentences. In addition, the detection mechanism of grammatical incongruence is attributed to resolving the conflicts with prediction.